The present invention relates generally to weight sensing modules, and more particularly, to torque compensated weight sensing modules having reduced sensitivity to torque in order to accurately measure weight in the presence of longitudinal and lateral torques.
Strain gauge transducers are widely used in weight measuring system to measure forces in scales, structures, mechanical structure, etc. The point of application of the weight is not always directly over the transducer, but may be laterally or longitudinally displaced relative to the transducer sensing axis. This results in a moment being applied to the transducer in addition to the measured shear force.
One such application is a crane arrangement is used to lift medical patients from a bed or chair and place them on a table, stool, other exam facilities, where their weights are needed for treatment purposes. The patients may be prone or upright. Their body profiles and their support harnesses produce great variation in the centers of gravity of the total masses. In addition, variable boom lengths and heights are needed to perform the position requirements. With the application of the techniques described herein, weight measurements accurate to 0.1% of weight measured are readily achieved. However, torque applied to the transducers often result in errors greater than 0.1%.
To show how torques contribute to weight inaccuracies, consider a typical strain gauge sensor or transducer depicted in prior art FIGS. 1a-1e. Please note that while a strain gauge type of sensing means is described, other sensing means employing capacitive, piezoresistive, or inductive techniques may be used. Bottom, side and top views of a metallic element (1) are shown in FIGS. 1a, 1b and 1c respectively. A weight force is applied to one end of the element and the opposite end is attached to a base or other structure. The inner section of the element is machined to produce narrowed down hinge points (4) to concentrate stresses resulting from the applied force. These stresses result in increased strains at these points and are readily measured with strain gauges (5), (6), (7), and (8). The element is deformed as shown in FIG. 1d as a result of the applied force and the tension and compression forces (9) that combine to create a moment to counteract the applied force. The strain gauge elements are conveniently connected as a Wheatstone Bridge as shown in FIG. 1e. An excitation voltage (10), either alternating or direct current voltage is applied to opposite corners of the bridge and the output signal (11) derived from the other opposite comers of the bridge are processed by an amplifier, analog to digital converter or other electronic means (12).
In practical applications, the force cannot always be applied to the end of the transducer as shown in prior art FIGS. 1a-1e. One such application is shown in prior art FIG. 2a. There, a lift mechanism raises medical patients and consists of a base (16), vertical mast (13), a moveable arm (14), sling arrangement (15), and a mechanical, electrical, or hydraulic actuator (17). In such applications, a force sensing module (18) senses the weight of the patient. The weight (19) is applied through a lever arm (20) that greatly increases the moment acting on the transducer.
Shown in prior art FIG. 2b, the transducer (21) now has a shear force (22) that is effectively applied to the end of the transducer (21) that shear force is opposed by tension stresses Ts and compressive stresses Cs. The weight (21) applied through lever arm (20) results in a moment (23) that is resisted by tension stresses Tt and compressive stresses Ct. Thus, the total stresses acting at each strain gauge is the sum of these stresses and the stress due to the moment can greatly exceed the measured stresses that are due to the shear force.
The electrical signals resulting from these stress additions are shown in prior art FIG. 2d. The signals from the shear forces (22) results in a differential (24) labeled as xe2x80x98+ signal out Sxe2x80x99 and xe2x80x98xe2x88x92 signal out Sxe2x80x99. The electrical signal from the moment results in a xe2x80x9ccommon modexe2x80x9d signal (26) and (27) labeled as xe2x80x98xe2x88x92 signal out T and 1 +signal out Txe2x80x99. Most of the common mode signal is rejected by the differential amplifier (12). However inconsistencies of manufacturing of the webs (4), strain gauge sensitivities, placement of gauges, linearity of deflections, etc. result in an output of transducer (21) that is influenced by lever arm (20). These factors influence the sensitivity of the transducers. The transducer (21) may have a calibration sensitivity that is positive or negative with respect to lever arm (20) increase. Typically this sensitivity may be in the order of xc2x11%.
Another moment is created when the weight applied is displaced laterally to the longitudinal as is depicted in prior art FIG. 2c. This is a top view of FIG. 2b and the weight is applied vertically (out of the paper). The result is a twist of the transducer. For some transducer designs, less sensitivity change due to the torque is obtained if the torque is applied as shown in prior art FIG. 3b as contrasted to that shown in prior art FIG. 3a. This is due to the somewhat less lever arm for the combined Ts and Tt, and Cs and Ct forces as shown in prior art FIG. 3c. 
Accordingly, it is a principle object of the present to invention to provide a force sensing module, with reduced sensitivity to torque in order to accurately measure weight in the presence of longitudinal and lateral torques, comprising two or more sensing means mounted vertically with output signal means combined and adjustable to minimize longitudinal torque response. The force sensing module further comprises a vertical support for lifting and supporting a load, a support arm coupled to the vertical support for engaging and holding a load, first and second sets of strain gauges, spaced from each other and either in vertical or horizontal alignment with each other. The strain gauges are connected at one end to the vertical support and at the other end to the arm for receiving a force proportional to the load and for receiving a torque from the arm. The force sensing module also comprises first and second Wheatstone bridges, wherein each Wheatstone bridge comprises one of the sets of strain gauges and at least one variable resistance coupled between the Wheatstone bridges with a resistors adjustable to reduce the sensitivity of the Wheatstone bridges to torque applied to the sets of strain gauges through the support arm.